Titanium aluminide (intermetallic compound of the Ti--Al series) are expected to be useful materials for internal-combustion engine components such as inlet and outlet valves and piston pins because they are light weight materials having superior rigidity and high temperature strength.
For practical applications to such heat resistant components, the material should have high oxidation resistance as well as high temperature strength. Titanium aluminides alone, however, do not have sufficient resistance to oxidation, so attempts have been made to improve the oxidation resistance by adding alloying elements.
For example, JP-A-1-246330 (the term "JP-A-" referred to herein signifies "unexamined Japanese patent publication") reports that the addition of 0.3.about.5.0% of Si to Ti-30.about.45 wt % Al improves the oxidation resistance. JP-A-1-259139 presents a Ti--Al intermetallic compound having superior high temperature oxidation resistance, containing 22.about.35 wt % of Al and 5.about.20 wt % of Cr, and it also notes that further improvement of high temperature oxidation resistance is achieved by adding 0.01.about.3 wt % of Y, 0.01.about.3 wt % of Re, 0.01.about.0.2 wt % of C, 0.01.about.1 wt % of Si, and 0.01.about.0.2 wt % of B. JP-B-1-50933 (the term "JP-B-" referred to herein signifies "examined Japanese patent publication") states that the addition of 100.about.1000 at PPM of P to a Ti--Al intermetallic compound composed of 40.about.50 at % of Ti and 60.about.50 at % of Al improves the oxidation resistance.
Nevertheless, the addition of these alloying elements does not necessarily result in a sufficient improvement of oxidation resistance, and furthermore, when a specific property is intended to be boosted, other superior characteristics often suffer bad effects.